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FORUMS General Gear Talk Flash and Studio Lighting 
Thread started 26 Aug 2006 (Saturday) 23:36
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Flash Photography 101, Chapter 4 - Guide Numbers and High Speed Sync

 
robbiewangyang
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Aug 04, 2007 17:47 as a reply to  @ post 3669252 |  #31

by the way, as a foreigner, i dont really know what does "chimp"mean mentioned in the chaptor of bouncing light. could anyone please explain that to me, thanks a lot.


:)what i have: Canon 20d, canon 18-55mm 3.5-5.6
canon 50mm f1.8 damaged:cry:
canon speedlite 430ex

:evil:what i want: sigma 17-35 f2.8 DG canon 70-200 f4L

  
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PacAce
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Aug 04, 2007 18:32 |  #32

robbiewangyang wrote in post #3669261 (external link)
by the way, as a foreigner, i dont really know what does "chimp"mean mentioned in the chaptor of bouncing light. could anyone please explain that to me, thanks a lot.

Maybe this will help:

https://photography-on-the.net …p?t=76476&highl​ight=chimp


...Leo

  
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robbiewangyang
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Aug 04, 2007 19:17 |  #33

Thanks mate. :D


:)what i have: Canon 20d, canon 18-55mm 3.5-5.6
canon 50mm f1.8 damaged:cry:
canon speedlite 430ex

:evil:what i want: sigma 17-35 f2.8 DG canon 70-200 f4L

  
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whiteflyer
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Aug 22, 2007 04:01 |  #34

I hope you don't mind , but for my own use I made a .pdf document of all 4 chapters. It is availabe at http://www.whiteflyer.​f2s.com/shared/files/F​LASH.pdf (external link)


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freefallu
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Sep 02, 2007 19:21 |  #35

Thanks for a great explanation! I have one question . you mention it better to close the apperture instead of using HSS. I am planning to take some pictures at 1pm when the sun is very high and researched HSS as i guessed it was something i would need given i would want to use fill flash. I am curious to know ( given you reccomend closing the apperture ) if you ultimately use a balance between ambient and HSS fill and how many stops below ambient you would expose on manual for ( then bringing up the subject with fill ) . I hope I have worded the question right.


Cheers David Cowman
Canon 5d, 400D , 24-105 L IS :: 70-200 f4 L :: 50 mm f1.4 :: Sigma 15mm f2.8 :: Canon 35 f1.4L :: Canon 85f1.2L 580EX x 2 ,ST - E2 , 2x Quantum turbo 2x2 batteries, Various flash devices from lumiquest and Stofen. Studio: 2 x Bowens 500 with lots of stuff to complement.

  
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Curtis ­ N
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Sep 02, 2007 19:42 |  #36

If flash distance range, recycle time and battery life are important, use normal flash, X-sync shutter speed and close the aperture to meter for the ambient. This will make most efficient use of the flash power.

If blurry backgrounds are important, set your aperture as you wish, adjust shutter speed to meter for the ambient, and use high speed sync.

For using fill flash when your subject is in direct sunlight, a little negative EC might be advised to avoid clipping the highlights. Maybe start at -1 EC (or adjust exposure manually for the same result) and go from there.

Chimp often, check the histogram and the "blinkies" on the LCD image, and adjust accordingly.


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Chris
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Sep 15, 2007 23:12 |  #37

I just bought a flash and I'm getting a headache trying to understand everything above. Next week is my daughters homecoming and I would like to get some nice outdoor shots of the group (maybe 4-6 people) I will be shooting around 5 pm. Can someone give me a simple explanation of how to add fill flash until I can understand all of this?


Chris

70D | 24-70 2.8 | 400 5.6 | 580 EXII | 2X Yongnuo 622C |

  
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René ­ Damkot
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Nov 29, 2007 06:23 |  #38

Came across this thread thru another one...

On the table: I think there is something strange going on there: Table two seems to assume a sync speed of 1/60s. The power output with HSS enabled is half of the maximum 'normal' output at 1/125s. This is not correct of course when your camera has a sync speed of 1/250s...
So, for a 1D2, the figures for HSS would actually be two stops better...


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jrsforums
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Nov 29, 2007 07:15 |  #39

René Damkot wrote in post #4406045 (external link)
Came across this thread thru another one...

On the table: I think there is something strange going on there: Table two seems to assume a sync speed of 1/60s. The power output with HSS enabled is half of the maximum 'normal' output at 1/125s. This is not correct of course when your camera has a sync speed of 1/250s...
So, for a 1D2, the figures for HSS would actually be two stops better...

Hi, Rene....

At whatever point the sync speed of the camera is exceeded, HSS will be needed to continue to use flash. WHile the calculation is dependent on the exact characteristics of the flash unit and sync "crossover", a good approximation is 2 stops of *maximum* flash output.

Note that this only comes into play (assuming TTL/ETTL) if you need the maximum output of the flash. If you do not need the maximum output, ETTL will adjust for it.

If you find that adding FEC or opening the aperture does not increase the flash results, you are probably pushing the max. output. If you have been above the sysnc speed (using HSS), you should be able to increase the output of the flash up to 2 stops by adjusting the settings to get at or below the sync speed.


John

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Curtis ­ N
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Nov 29, 2007 08:44 |  #40

René Damkot wrote in post #4406045 (external link)
Came across this thread thru another one...

On the table: I think there is something strange going on there: Table two seems to assume a sync speed of 1/60s. The power output with HSS enabled is half of the maximum 'normal' output at 1/125s. This is not correct of course when your camera has a sync speed of 1/250s...
So, for a 1D2, the figures for HSS would actually be two stops better...

As John points out, the shutter speeds in Table 2 at or slower than your camera's X-sync speed do not apply, since there's no way to use HSS at those speeds. The flash unit is programmed to default to normal flash when the shutter speed allows.

HSS emits what amounts to a continuous light source for the duration of shutter curtain movement. It has to turn on when the first curtain starts to open and remain on until the second curtain is fully closed.

As you observed, extrapolating the table 2 data would indicate that 1/60 shutter speed would give you guide numbers to match the normal flash numbers in table 1.

This could mean that the Sigma flash (on which the table is based) emits light for 1/60 second in HSS mode, basically spreading out the energy in its capacitor over that period of time. Or perhaps it emits light for a somewhat shorter period of time but looses some efficiency in the process.

While the Canon manuals do not have a similar table for HSS, I know by observing the distance scale on my 580EX II that using HSS basically cuts the effective range in half, compared to normal flash at 1/250. In other words, the indicated distance at 1/500 f/8 will be half that of 1/250 f/11.

It is true that faster X-sync speeds allow for better flash range when competing with ambient light, since faster shutter speeds allow for wider apertures. Thus, the faster your camera's X-sync speed is, the more range you will sacrifice with HSS.


"If you're not having fun, your pictures will reflect that." - Joe McNally
Chicago area POTN events (external link)
Flash Photography 101 | The EOS Flash Bible  (external link)| Techniques for Better On-Camera Flash (external link) | How to Use Flash Outdoors| Excel-based DOF Calculator (external link)

  
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René ­ Damkot
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Nov 29, 2007 08:48 |  #41

I think I know the basics ;)

I would have expected flash output to be half of 'maximum reach' at twice sync speed. (not sure why I'd expect that, now that I think of it :o)

Okay, gave it a try: 580 ex at max power.
HSS enabled, ISO 100, F/4. Zoom 105mm.

1/250s and slower: 18m (GN 72 (m))
1/320s - 1/500s: 6m (GN 24 (m))
After that it goes lineair:
1/640s to 1/1000: 4m (GN 16)
1/1250 to 1/2000: 3m (GN 12)
1/25000 to 1/4000: 2m (GN 8)
1/5000 to 1/8000: 1,5m (GN 6)

Still, I say the table is incorrect, since it should start at sync speed. So 1/125s; 1/160s and 1/180s (should be 1/200s) have no meaning.


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René ­ Damkot
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Nov 29, 2007 08:48 |  #42

Curtis N wrote in post #4406534 (external link)
This could mean that the Sigma flash (on which the table is based) emits light for 1/60 second in HSS mode, basically spreading out the energy in its capacitor over that period of time. Or perhaps it emits light for a shorter period of time but loose some efficiency in the process.

Good point.
Seems we agree ;)


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Curtis ­ N
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Nov 29, 2007 08:59 |  #43

René Damkot wrote in post #4406554 (external link)
Still, I say the table is incorrect, since it should start at sync speed. So 1/125s; 1/160s and 1/180s (should be 1/200s) have no meaning.

That flash unit is built in several versions for different brands and models of camera bodies, so the table covers the range of those bodies with slower sync speeds. You are correct that the shutter speeds in that table at or slower than the X-sync speed of whatever it's attached to are meaningless.

René Damkot wrote in post #4406556 (external link)
Seems we agree ;)

I find that smart people tend to agree with me quite often.


"If you're not having fun, your pictures will reflect that." - Joe McNally
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jrsforums
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Nov 29, 2007 22:22 |  #44

René Damkot wrote in post #4406554 (external link)
I think I know the basics ;)

I would have expected flash output to be half of 'maximum reach' at twice sync speed. (not sure why I'd expect that, now that I think of it :o)

Okay, gave it a try: 580 ex at max power.
HSS enabled, ISO 100, F/4. Zoom 105mm.

1/250s and slower: 18m (GN 72 (m))
1/320s - 1/500s: 6m (GN 24 (m))
After that it goes lineair:
1/640s to 1/1000: 4m (GN 16)
1/1250 to 1/2000: 3m (GN 12)
1/25000 to 1/4000: 2m (GN 8)
1/5000 to 1/8000: 1,5m (GN 6)

Still, I say the table is incorrect, since it should start at sync speed. So 1/125s; 1/160s and 1/180s (should be 1/200s) have no meaning.

Rene, I am not sure what you are driving at. However, here is the HSS chart from the 550EX, which should be close to the 580EX...


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jrsforums
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Nov 29, 2007 22:47 as a reply to  @ jrsforums's post |  #45

About 1 1/2 years ago, I had asked Doug Kerr the effective difference between 'X'-sync and HSS (FP) mode. He had posted this info on 'ProPhoto" forum, but does not appear to be available now. I am sure Doug would not mind, so here is an excerpt of his reply...

"...It is difficult to assess the "total luminous output" of the FP ("HS sync") burst, since we don't necessarily know its actual duration

In any case, what is of interest is the effect on exposure of the FP burst (for some shutter speed of interest), which we may wish to compare with the effect on exposure of the X burst (for any workable shutter speed) - all for any consistent aperture and ISO sensitivity.

And the guide number tables in the 550EX manual suggests that this quantity indeed varies linearly with the exposure time (which is what we might think). It presumably wouldn't continue to increase linearly for exposure times above the duration of the burst, but it is very linear for times up to 1/125 sec (the longest time listed in the table).

The tables suggests that the (maximum) exposure impact of the FP burst, with a shutter speed of 1/125 sec (of course we can't try that on a 20D) is about 1 stop less than that of the X burst (at any shutter speed).

With a shutter speed of 1/320 sec (the slowest for which we can get an FP burst on a 20D), the maximum exposure impact of the FP burst is about 2.35 stops less of that of the X burst (at any shutter speed)...

...Lets consider shots on a 20D using two shutter speeds , 1/250 sec (at which we will have the X burst) and 1/320 sec (at which we will have the FP burst).

[Recall that we cannot make the FP burst happen at 1/250 sec, and we cannot make the X burst happen at 1/320 sec.]

According to the guide number table, the total exposure impact for a 1/320 sec exposure (FP burst) is 2.35 stops less than that for an X burst exposure.

If we want the ambient exposure to be the same in the two shots, the aperture for the 1/320 sec case will need to be 0.35 stop larger than for the 1/250 sec case.

Then, the flash exposure for the 1/320 sec FP case will be about 2 stops less than for the 1/250 sec X case (+0.35 - 2.35 = -2.00)

Now let's consider a shutter speed of 1/500 sec. There, according to the table, the exposure impact of the FP burst would be 3 stops less than the impact of the X burst. If we want the ambient exposure to be the same as for the 1/250 sec shot, the aperture at 1/500 sec will need to be 1 stop larger.

Then, the flash exposure for the 1/500 sec FP case will (again) be about 2 stops less than for the 1/250 sec X case (+1 - 3 = -2.00)

So if the issue is the maximum available flash exposure, for a given ISO sensitivity and aperture, exposure, the X burst shot at 1/250 sec will win over the FP burst shot (at any shutter speed above the X-sync limit) every time, always by 2 stops..."

Later he followed with:

"...In further regard to our recent discussion of the use of the FP and X bursts in flash fill operation,. I thought it would be interesting tom look, at an ideal theoretical basis, at the matter of obtaining sufficient flash exposure in a fill situation.

Let's assume a situation under sunlight illumination of the level assumed by the "sunny 16" rule. Then one set of expose parameters that should produce appropriate exposure (with no flash yet involved) would be:

ISO sensitivity: ISO 100
Shutter speed: 1/100 sec
Aperture: f/16

Then assume we want to attain a flash fill ratio of 1:1 on our main subject, while keeping the same overall exposure of the main subject. First, we must reduce the ambient exposure to 1/2 its original value. Suppose we want to retain the ISO sensitivity and aperture. Then this would call for a new shutter speed of 1/200 sec (still within the X-sync limit).

Now, let's see at what distance to the subject could the flash play its role. We will assume a 550EX, with its head at the "50 mm" position. At ISO 100, the guide number would be 42 m (138 ft), and at f/16, the "reach" would be 8.6 feet (for proper exposure of the subject by flash illumination alone. But here, we only want the flash to contribute half the exposure, so that reach distance would become 12.2 feet (8.6 * sqrt 2).

Thus, under the assumed conditions, for a main subject at 12.2 feet we would indeed be pressing the flash to its maximum capability.

Now, suppose a brighter situation would indicate a "full" exposure, of, say, f/16 at1/400 sec, calling for use of the FP burst. Our actual aperture (for "1/2 normal" ambient exposure, because the flash fill situation) would be f/11. The GN (FP burst, ISO 100, "50 mm" head position) would be 16.6 m (54.4 ft), which at f/11 would give a subject distance (for exposure by flash only) of about 5 feet. With the flash only asked to contribute half the exposure, the maximum flash distance to the main subject would be about 7 feet!

I can see why your concern with available flash output in this situation.

If we instead decided to use an ambient exposure (1/2 normal) of f/20 at 1/250 sec (X burst, with a GN of 138 ft), then the maximum flash distance to the main subject (with the flash contributing 1/2 normal exposure) would be about 9.7 feet..."

Again, he followed with:

"...According to the tables in the 550EX manual, the effect is just what we have for ambient exposure: the exposure impact with the FP burst in effect is directly proportional to exposure time. (This linear relationship of course would not continue for shutter times longer than the entire duration of the FP burst sequence.)

This is in fact why the "1/2 disadvantage" in fill flash reach with the FP burst is the same for any shutter speed where the FP burst is used. That is, as we raise the shutter speed, if we change the aperture to maintain the same ambient exposure, with the FP burst in effect, this changes the flash exposure by the same amount, resulting in a constant flash exposure (for a given flash "output", including the maximum available) as well as a constant ambient exposure..."

This all is probably more than most want, or need, to read, but I thought some would find it interesting.


John

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Flash Photography 101, Chapter 4 - Guide Numbers and High Speed Sync
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